Once it was asked in whispers, or with winks. The timid among us, though undeniably curious, feared raised eyebrows. Jokes about little green men. Who could take such a question seriously, yank it from the misty realms of science fiction and drop it under the searchlight of science? Well, our national space agency, for one. What's more, NASA seems pretty confident these days about the answer: Astrobiology, as defined on an official agency Web site, is "the study of the living universe."

They can't talk. They're not measurably intelligent. They can't even move on their own. Yet Katherine Freeman of Penn State University's geosciences department has been learning something from common marine algae.

"We're trying to look back in time," says Freeman, "to see if the level of carbon dioxide in the atmosphere was higher when the temperature was higher." By understanding Earth's ancient atmosphere, scientists hope to predict the consequences of modern-day greenhouse gas emissions.